This present invention relates generally to devices for heating air. More specifically, the invention discloses an apparatus for heating air in a space heating system. The apparatus of the invention is particularly suitable for use in heating, ventilation and air conditioning (HVAC) systems for commercial buildings, but is adaptable for use in residential applications.
HVAC systems for commercial buildings such as stores, manufacturing facilities, low rise office buildings and similar structures typically contain "package" units which combine air conditioning, heating and sometimes air handling equipment in a single housing. Such package units are generally installed outside the building envelope, frequently at ground level or on the building roof. A typical package unit comprises: an air conditioning compressor, associated condenser and evaporator coils and air conditioning system auxiliary components; a heating system comprised of some combination of heat source and heat exchange means such as electrical resistance heaters or gas or oil fired burners and associated heat exchangers; and a flow path for air to be cooled or heated to pass through the unit. Air from the building is circulated through the unit and returned to the building by means of a system of supply and return ductwork, with some means of causing a flow of air through the system. The air handling equipment for moving air through the system may be separate or included in the package unit. Architectural, engineering, economic and environmental considerations have driven efforts to reduce the size and weight of package units as well as other types of HVAC equipment, to improve the thermal efficiency of heating systems and, in the case of heating systems that produce heat by burning a fuel, to reduce the emission of potential pollutants.
A heat pipe is a heat transfer device capable of transferring heat from a higher to a lower temperature at a high rate. The principles of the construction and operation of heat pipes are well known in the art. Because of their high thermal conductance, heat pipes are well adapted to use in HVAC equipment for transferring heat from the heat source to the air to be heated and can result in significant reductions in space and weight reductions in HVAC heating components, for a given heat transfer rate, over other types of heat transfer devices such as conventional flue gas to air heat exchangers.
The usual means by which a gaseous or vaporized liquid fuel is burned in a space heating appliance is a ribbon type burner located under the heat exchanger or by what is known in the industry as an "inshot" or jet type burner whose flame is directed into the inlet of the heat exchanger. Natural convection or a blower to create a forced draft are generally used to cause the gases of combustion to flow through and out of the heat exchanger. In such a furnace, the burning process generates not only heat but low frequency sound termed "combustion roar." If the appliance is connected into an air ducting system, the ducting tends to amplify the sound and transmit it to spaces remote from the appliance.
The products of combustion from flame burners in space heating appliances now in widespread use contain oxides of nitrogen (NO.sub.x). The oxides are vented to the atmosphere with the combustion products as flue gases. Limiting the concentration of NO.sub.x in the flue gases is desirable, as NO.sub.x can contribute to air pollution and appliances sold in certain jurisdictions must comply with very low NO.sub.x emission requirements.
The use of a radiant infrared burner in a heating device instead of the more usual flame burner has certain advantages, chief among them as pertain to this invention are that the gases of combustion produced by a radiant burner have very low concentrations of NO.sub.x and that the radiant burner burns silently, without producing combustion roar.
A conventional gas or oil fired space heating device, such as is now in widespread use, operates at a relatively low thermal efficiency, converting typically only 65 to 70 percent of the available heat energy in the fuel burned to heat available to warm the space it serves. Much of the efficiency loss is due to the heat contained in the hot gases of combustion produced by the burner that pass "up the flue" and are lost to the atmosphere. The efficiency of such a space heating appliance can be significantly improved by condensing certain of the gases of combustion produced thus extracting the latent heat of condensation from those gases before they escape "up the flue."